The present invention relates to a thermal transfer printing method of making an initial character image remaining on a spent ink ribbon illegible and a thermal transfer printing apparatus carrying out the above method.
With a heavy usage in this art, there is a thermal transfer printing apparatus that allows a thermal head having a plurality of heating resistive elements arranged in a main scan direction to transfer information to be printed, such as image information and character information, from a strip-shaped ink ribbon to a recording paper (or an intermediate transfer film) while feeding the ribbon and the paper (intermediate transfer film) in piles. In the printing apparatus, the ink ribbon has a strip-shaped ribbon base and a fusible or sublimation multicolor ink layer applied on the ribbon base. The multicolor ink layer consists of respective ink layers in yellow (Y), magenta (M), cyan (C) and black (BK) which are applied on the ribbon base repeatedly and respectively compartmentalized to have a predetermined size each in accordance with the recording paper (intermediate transfer film).
In this kind of thermal transfer printing apparatus, generally, sublimation dyes are used for respective colors yellow (Y), magenta (M) and cyan (C). In the thermal transfer operation, since such colors' transferred (or re-transferred) traces are remaining in the ink ribbon and the intermediate transfer film indistinctly, it is impossible for a third party to make out image information from these traces. Additionally, as these colors are mainly used for printing various images, they have a reduced degree of information secrecy in comparison with that of character information.
On the contrary, fusible pigments are generally used for black (BK) layers in the ink ribbon for purposes of printing of character information and bar-codes. Since such fusible pigments' transferred traces or re-transferred traces (reversed image) are remaining in the ink ribbon and the intermediate transfer film distinctly, it is possible for a third party to make out image information from these traces. It is especially noted that the character information contains information in high degree of secrecy frequently.
As for character information printed in black (BK), therefore, there is a fear of leakage of confidential information due to stolen spent ink ribbons and spent intermediate transfer films. When disposing of these spent ribbons and films, we have to apply any special treatment on them for preservation of confidentiality.
Japanese Patent Laid-Open Publication No. 2002-211064 discloses a transfer type image recorder capable of making initial images (initial image data) remaining on a spent ink ribbon illegible easily.
FIGS. 1A to 1E illustrate an initial image (initial image data), overwrite image data and a superimposed image (superimpose image data) respectively to explain the operation of making the initial image remaining on a spent ink ribbon illegible by the above transfer type image recorder.
In the transfer type image recorder of the publication, after transferring ink from the ink ribbon to a recording paper by a heat sensitive head while pinching the ink ribbon and the recording paper between the head and a platen roller, the heat sensitive head overwrites different overwrite image data B1 (or B2) on the remaining initial image (initial image data) A to produce a superimpose image (superimpose image data) C1 (or C2), making the initial image A on the spent ink ribbon illegible, as shown in FIGS. 1A to 1E.
More concretely, FIG. 1A illustrates one example of the initial image A, FIG. 1B one example of the overwrite image data B1 having a random character row, and FIG. 1C illustrates one example of the superimpose image C1 obtained by superimposing the image data B1 on the initial image A.
As obvious from FIG. 1C, it is almost impossible to make out the initial image A in the superimpose image C1.
Then, the overwrite image data B1 is generated with use of random character rows including numerals, alphabets, kana, kanji, etc. on the ground of e.g. JIS (Japanese Industrial Standards). Further, as the overwrite image data B1 is overwritten upon turning over the ink ribbon, the resulting superimpose image C1 comprises the initial image A and the upside-down overwrite image data B1 on the ink ribbon, as shown in FIG. 1C. Thus, it is almost impossible to make out the initial image A in the superimpose image C1.
Besides the random character rows, the above publication discloses the generating of overwrite image data B2 with use of relatively simple graphic symbols, such as kinked line and broken line (not shown), as shown in FIG. 1D. FIG. 1E illustrates a superimpose image C2 where the overwrite image data B2 is overlaid on the initial image A. In connection, the publication has a statement that it is almost impossible to make out the initial image A in the superimpose image C2.